SiC substrate and method of manufacturing the same

ABSTRACT

A method of manufacturing a SiC substrate which has a first principal surface and a second principal surface, includes the step of removing, by a vapor phase etching process, at least a portion of a work-affected layer which is formed by mechanical flattening or cutting on the first principal surface of the SiC substrate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a SiC (silicon carbide)substrate and a method of manufacturing the SiC substrate and, moreparticularly, to a method of manufacturing a SiC substrate in which atleast one surface is polished.

[0003] 2. Description of the Related Art

[0004] Recently, there has been a growing demand for lasers and lightemitting diodes which use GaN-base semiconductors as light emittinglayers and can emit light having a short wavelength, such as theultraviolet region and blue color. These types of lasers and lightemitting diodes are expected to be used as light sources for recordinginformation at high recording densities on optical disks and reproducinginformation therefrom or light sources for displaying images in fullcolor or for use in illumination. In general, it is difficult to cause aGaN-base semiconductor to grow into the shape of a large single crystalingot having few crystal defects. For this reason, techniques forepitaxially growing a GaN-base semiconductor layer on a sapphire singlecrystal substrate or a SiC single crystal substrate are receivingattention and a sapphire single crystal substrate or a SiC singlecrystal substrate on which a GaN-base semiconductor layer is to beformed is sought after.

[0005] A SiC single crystal substrate is demanded also as a substratefor forming a high-quality SiC semiconductor layer. Because a SiCsemiconductor has a wide band gap, a large dielectric breakdown electricfield and a large thermal conductivity in comparison with a GaAssemiconductor, research and development have been carried out to formhigh-quality SiC semiconductor layers on a SiC single crystal substrateand to realize semiconductor elements operating at high temperatures andpower semiconductor elements having a high breakdown voltage. Inaddition, in the semiconductor process, dummy wafers made of SiC aredemanded because these wafers have excellent heat resistance, highthermal conductivity, high-temperature strength, low thermal expansion,wear resistance, etc.

[0006] A sapphire single crystal substrate or a SiC single crystalsubstrate for such applications is required to provide high workingaccuracy in the flatness of the substrate, the smoothness of thesubstrate surface, etc. However, generally a sapphire single crystal orSiC has high hardness and excellent corrosion resistance, and hence theworkability of manufacturing such a substrate is bad and it is difficultto obtain a sapphire single crystal substrate and a SiC substrate havinghigh working accuracy.

[0007] In particular, as described in Japanese Laid-Open PatentPublication No. 55-20262, when an ingot of sapphire single crystal iscut and lapped and its surface is then mirror finished, a work-affectedlayer in which work strains have been generated remains on the backsurface, posing the problem that the substrate warps.

[0008] For this reason, when photolithography is performed on such asubstrate, there arises some problems in that it becomes impossible toperform vacuum chucking of the substrate by an exposure device, etc.,and that the accuracy of exposure worsens due to a poor flatness of thesubstrate, and so on. Furthermore, when a thin layer of metal, ceramics,etc. is formed on such a substrate in which a work-affected layerremains, the problem that the substrate breaks because of the additionof the stresses of the thin film to the residual stresses of thesubstrate arises.

[0009] For this reason, the Japanese Laid-Open Patent Publication No.55-20262 discloses a technique which involves immersing a sapphiresingle crystal substrate in heated phosphoric acid or potassiumhydroxide solution and removing a work-affected layer remaining in thesubstrate by dissolving the work-affected layer thereby to eliminate awarp of the substrate.

[0010] However, in the case of a SiC substrate, it is impossible todissolve SiC with heated phosphoric acid or potassium hydroxidesolution. Although fused alkalis which are heated to not less than 300°C. are known as solutions which dissolve Sic, large-scale equipment isnecessary for safely handling high-temperature fused alkalis.

[0011] The Japanese Laid-Open Patent Publication No. 55-20262 disclosesthat ion sputtering and ion etching may also be adopted as otherprocesses for removing the work-affected layer of a sapphire singlecrystal substrate. However, these processes involve performing theetching of a substrate surface by utilizing the physical energy of ionsof argon, etc., which are accelerated by causing these ions to collideagainst the substrate surface. Thus, these processes have the problemthat the etching rate is low.

[0012] Furthermore, because the melting point of SiC is not less than2000° C., it is necessary to heat a SiC substrate to not less than 1600°C. in order to remove work strains by annealing. Large-scale equipmentis necessary for subjecting the SiC substrate to heat treatment at sucha high temperature.

SUMMARY OF THE INVENTION

[0013] In order to solve the problems described above, preferredembodiments of the present invention provide a method of manufacturing aSiC substrate in which a work-affected layer is removed under practicalconditions.

[0014] According to a first preferred embodiment of the presentinvention, a method of manufacturing a SiC substrate which has a firstprincipal surface and a second principal surface, includes the steps offorming a work-affected layer by mechanical flattening or cutting on thefirst principal surface of the SiC substrate, and removing, by a vaporphase etching process, at least a portion of the work-affected layerwhich is formed by mechanical flattening or cutting on the firstprincipal surface of the SiC substrate.

[0015] It is preferable that the vapor phase etching process is areactive ion etching process.

[0016] The second principal surface is preferably a surface where anelement is to be formed.

[0017] According to another preferred embodiment of the presentinvention, the method described above further includes a step of mirrorpolishing the second principal surface.

[0018] According to another preferred embodiment of the presentinvention, in the method described above, the SiC substrate has awork-affected layer which is formed by mechanical flattening or cutting,on the second principal surface, and the method also includes the stepsof removing at least a portion of the work-affected layer of the secondprincipal surface by a vapor phase etching process, and mirror polishingat least the second principal surface after the steps of removing areperformed.

[0019] In this method, the SiC substrate preferably has a work-affectedlayer which is formed by mechanical flattening or cutting, on the secondprincipal surface, and the method further includes the step of removingthe work-affected layer of the second principal surface by mechanicalpolishing and chemical mechanical polishing and mirror finishing thesecond principal surface.

[0020] In the methods described above, the first principal surfaceobtained by the step of removing preferably has a surface roughness ofabout 10 nm to about 1 μm.

[0021] In addition, the method described above also preferably includesa step of cutting the SiC substrate from an ingot of SiC and the firstprincipal surface and second principal surface are formed by the step ofcutting.

[0022] Also, in the step removing, the SiC substrate is preferably heldso as to allow a change in the warp of the SiC substrate.

[0023] It is preferred that a gas containing fluorine is used in thevapor phase etching process. The gas containing fluorine is preferablyCF₄ or SF₆.

[0024] In addition, in the vapor phase etching process described above,the work-affected layer is preferably removed at an etching rate in arange of about 0.5 μm/hr to about 20 μm/hr.

[0025] The SiC substrate is preferably one of amorphous, a poly crystaland a single crystal.

[0026] Yet another preferred embodiment of the present inventionprovides a SiC substrate manufactured according to a method including astep of removing, by a vapor phase etching process, at least a portionof a work-affected layer which is formed by mechanical flattening orcutting on the first principal surface of the SiC substrate.

[0027] An additional preferred embodiment of the present inventionprovides a SiC substrate including two substantially parallel principalsurfaces, wherein only one of the two principal surfaces is mirrorfinished and the warp is not more than about ±50 μm.

[0028] Other features, elements, characteristics, steps and advantagesof the present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a schematic view showing how a substrate is cut from aSiC ingot.

[0030]FIG. 2 is a sectional view showing work-affected layers formed ina substrate cut by machining.

[0031]FIG. 3A shows a SiC sheet formed by sintering and FIGS. 3B and 3Ceach show a SiC substrate fabricated by a mechanical plane working fromthe SiC shown in FIG. 3A.

[0032]FIGS. 4A to 4D are each a sectional view to explain a method offabricating a SiC substrate according to a first preferred embodiment ofthe present invention.

[0033]FIG. 5 is a sectional view showing the state of a SiC substrateheld in a substrate holder of a reacting ion etching device.

[0034]FIGS. 6A to 6C are each a sectional view to explain a method offabricating a SiC substrate according to a second preferred embodimentof the present invention.

[0035]FIGS. 7A to 7C are each a sectional view to explain a method offabricating a SiC substrate according to third preferred embodiment ofthe present invention.

[0036]FIG. 8 is a graph showing the relationship between the etchedamount by reactive etching and the flatness of a substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] In various preferred embodiments of the present invention, awork-affected layer formed on a SiC substrate by mechanical flatteningor cutting is removed by a vapor phase etching process. In particular,it is preferable to use a reactive gas in the vapor phase etchingprocess. For example, an ion etching process using a reactive gas orreactive ion etching (RIE) can be used in preferred embodiments of thepresent invention and it is more preferable to use reactive ion etchinghaving high chemical reactivity.

[0038] In the field of manufacturing semiconductor devices, a method ofremoving thin films such as a semiconductor film and an insulating filmby reactive ion etching has been known. In this field, however, reactiveion etching is used in the patterning and etching of thin films formedby use of a thin-film forming device or in the removal of oxide films onthe surface of a semiconductor substrate, and the etched amount istypically not more than several hundreds of nanometers. Furthermore, itis known that when the reactive ion etching process is used, damage by aplasma is apt to occur in a semiconductor layer. For this reason, in acase where damage by a plasma poses a problem, it has been generalpractice to remove a semiconductor layer or an insulating layer by thewet etching process using an etching liquid or to remove a semiconductorregion where damage by reactive ion etching has occurred due to reactiveion etching after the etching by the reactive etching process. That is,in a step where the use of wet etching is desirable, etching by reactiveion etching is often an inappropriate process.

[0039] In spite of this background, the present inventors discoveredthat a SiC substrate can be etched or lapped at a practical etching rateby vapor phase etching, preferably, by reactive ion etching using a gasincluding fluorine. The idea of lapping a SiC substrate, which is not athin film, by vapor phase etching on the order of several microns hasnot been proposed or performed in the field of the manufacturing ofsemiconductor devices. Thus, one of the unique characteristics of thepresent invention is in removing a work-affected layer formed on asurface that is opposite to a surface on which a semiconductor elementis to be formed by vapor phase etching. As will be described in detailbelow, even if warping occurs on a SiC substrate at this time, thework-affected layer can be etched almost uniformly from the surface andthe warp of the SiC substrate is eliminated in association with theremoval of the work-affected layer. Therefore, it is possible tomanufacture a SiC substrate that has excellent parallelism and TTV(total thickness variation) of the substrate. According to preferredembodiments of the present invention, the warp of a SiC substrate havinga diameter of not more than about 4 inches can be reduced to withinabout ±50 μm. No SiC substrate having such a small warp has beenobtained by conventional manufacturing methods.

[0040] Furthermore, even when reactive ion etching is used to remove awork-affected layer formed on a SiC substrate, damage by reactive ionetching which occurs in the SiC substrate does not cause a problem. Thisis because a principal surface from which a work-affected layer is to beremoved is a surface that is opposite to a surface which is to be mirrorpolished and on which a semiconductor element is to be formed.Alternatively, a principal surface from which a work-affected layer isto be removed is a surface which can be further subjected to mirrorpolishing after the removal of the work-affected layer by reactive ionetching.

[0041] The method of manufacturing a SiC substrate according topreferred embodiments of the present invention will be specificallydescribed in the following. As shown in FIG. 1, a SiC substrate 1 usedin preferred embodiments of the present invention is preferably a cutpiece which is cut from an ingot 2 of SiC. The ingot 2 of SiC may besingle crystal, polycrystal or amorphous. The ingot 2 of SiC may includeadditional elements such as Al, Zr, Y and O other than Si and C orsubstituent elements. It should be construed that in this specification,a SiC substrate includes a SiC substrate including SiC which may includeadditive elements or constituent elements.

[0042] The shape of the SiC substrate 1 is not especially limited andSiC Substrates of various sizes, thicknesses and plane shapes can beused. For example, in a case where a SiC substrate 1 consisting of asingle crystal is used as a substrate for the epitaxial growth of aGaN-base semiconductor layer, a disk-shaped SiC substrate 1 having adiameter of about 2 inches and a thickness of about 500 μm is preferablyprepared.

[0043] For the cutting of the ingot 2 of SiC, it is possible to use acutting blade, which is an outside peripheral cutting edge or an insideperipheral cutting edge, a wire saw, or other suitable device. The SiCsubstrate 1 cut by such cutting includes, as shown in FIG. 2,work-affected layers 3 a, 3 b in the vicinity of a first principalsurface 1 a and a second principal surface 1 b formed by cutting. Inthis specification, cutting refers to the cutting by the cutting bladeof the outside peripheral cutting edge or the inside peripheral cuttingedge, the cutting by the wire saw described above, or other suitablecutting apparatus.

[0044] Work strains are caused in the work-affected layers 3 a, 3 b dueto mechanical cutting. For this reason, compressive stresses which mightmake both the first principal surface 1 a and the second principalsurface 2 b convex act on the work-affected layers 3 a, 3 b. Themagnitude of compressive stresses depends on the thickness of thework-affected layers 3 a, 3 b. As is apparent from FIGS. 1 and 2,because the first principal surface 1 a and second principal surface 2 bof the SiC substrate 1 are formed by mechanical cutting under the sameconditions, the thickness of the work-affected layer 3 a and thework-affected layer 3 b is substantially equal. For this reason, thecompressive stresses acting on the work-affected layer 3 a and thework-affected layer 3 b become equal, with the result that scarcely anywarp occurs in the SiC substrate 1 cut from the ingot 2 of SiC as awhole. Although the thickness of the work-affected layers 3 a, 3 bdepends on cutting conditions, such as a cutting method, and theproperties of a substrate, it is said that generally this thickness isabout 3 to about 10 times the maximum surface roughness Rmax of asurface formed by cutting.

[0045] In FIGS. 1 and 2, the SiC substrate 1 cut from the ingot 2 of SiCwas described. However, a SiC substrate used in various preferredembodiments of the present invention may be obtained by thinning a SiCsheet, which is formed by sintering. As shown in FIG. 3A, a SiC sheet 4formed by sintering is prepared and subjected to mechanical flatteningby polishing at least either of the first principal surface 4 a and thesecond principal surface 4 b by use of a lapping device or othersuitable device. By performing mechanical flattening until the thicknessof the SiC sheet 4 becomes a desired value, the SiC substrate 4′ shownin FIG. 3B is obtained. In the SiC substrate 4′, only its secondprincipal surface 4′b is formed by mechanical polishing and awork-affected layer 3 b is formed by mechanical polishing in thevicinity of the surface of the second principal surface 4′b. Because thefirst principal surface 4 a is a surface of the SiC sheet 4 formed bysintering, no work-affected layer 3 b is formed on the first principalsurface 4 a. For this reason, in the SiC substrate 4′, the secondprincipal surface 4′b is warped to provide a convex state undercompressive stresses due to the work-affected layer 3 b.

[0046] In this specification, mechanical flattening refers to polishingby a lapping device by use of an abrasive, polishing by a verticalgrinder, or other suitable apparatus. In a case where a work-affectedlayer is present in the vicinity of the surface of a principal surfaceof a substrate, the work-affected layer is removed by polishing thesubstrate by mechanical flattening. However, work strains are alwaysgenerated in a region near the surface of a principal surface of thesubstrate and a new work-affected layer is formed. As a result, awork-affected layer is always present on a principal surface of thesubstrate subjected to mechanical flattening. As described above, thethickness of this work-affected layer depends on the maximum surfaceroughness Rmax of the surface. A surface polished by mechanicalflattening has a surface roughness Ra of about 10 nm to about 1 μm.

[0047] As shown in FIG. 3C, in a case where the first principal surface4 a and second principal surface 4 b of the SiC sheet 4 are mechanicallypolished, the SiC substrate 4′ in which the work-affected layers 3 a, 3b are formed on the first principal surface 4′a and the second principalsurface 4′b is obtained. As described above, the thickness of thework-affected layer 3 b depends on the maximum surface roughness Rmax ofthe first principal surface 4′a and the second principal surface 4′b.For this reason, regardless of the polished amount of the firstprincipal surface 4 a and second principal surface 4 b, the thickness ofthe work-affected layer 3 a and work-affected layer 3 b becomes almostequal. Generated compressive stresses are almost equal on the side ofthe first principal surface 4′a and the side of the second principalsurface 4′b, and scarcely any warp occurs in the SiC substrate 4′ shownin FIG. 3C.

[0048] Next, the step of removing a work-affected layer 3 by thereactive ion etching process will be described. Various reactive ionetching devices used in the semiconductor manufacturing process, such asa parallel flat plate type reactive ion etching device, an ECR (ElectronCyclotron Resonance) reactive ion etching device and an ICP (InductivelyCoupled Plasma) etching device can be used as the device used in thereactive ion etching process. It is desirable to use a gas including Fin etching. Although it is possible to use F₂, CF₄, CHF₃, CH₂F₂, CH₃F,SF₆, etc., it is more preferred to use CF₄ or SF₆. A mixed gas obtainedby adding other gasses such as Ar, H₂, O₂ and N₂ to a gas including Fmay be used.

[0049] The SiC substrate 1 is held in a substrate holder in such amanner that the work-affected layer 3 to be removed is exposed within achamber of a reactive ion etching device. At this time, it is preferredthat the whole of the SiC substrate 1 is not bonded and fixed to thesubstrate holder so that the substrate holder can hold the SiC substrate1 by allowing a change in the warp even when the warp of the SiCsubstrate 1 changes during etching.

[0050] The magnitude of power to be input, the gas pressure during areaction and the flow rate of a reactant gas depend on the type of adevice to be used, the crystallization state of a SiC substrate to beetched and the number of SiC substrates to be introduced at a time. Itis preferable to adjust these parameters so that the etching rate forthe removal of a work-affected layer becomes about 0.5 μm/hr to about 20μm/hr. When the etching rate is lower than about 0.5 μm/hr, the etchingefficiency is low and there is a problem in the process capability. In ageneral reactive etching device, it is difficult to increase the etchingrate to rates higher than about 20 μm/hr. Practically, it is morepreferred to cut a work-affected layer at an etching rate of about 1μm/hr to about 5 μm/hr.

[0051] By the reactive ion etching process, a work-affected layer of theSiC substrate 1 reacts chemically with the chemical species in anetching gas and becomes a gas, which is removed. By using reactive ionetching, a work-affected layer is removed with the surface conditionthat exists before etching being kept as it is. Therefore, the surfaceroughness of the substrate surface is substantially maintained beforeand after the reactive ion etching.

[0052] Because the removal of a work-affected layer by the reactive ionetching process proceeds by the contact of the surface of thework-affected layer with an etching gas, it proceeds substantiallyuniformly from the surface of the work-affected layer even when the SiCsubstrate 1 is warped and hence the thickness of the work-affected layerdecreases uniformly as a whole. Stresses by work-affected layer decreasewith decreasing thickness of the work-affected layer and the warp of theSiC substrate 1 is eliminated. When the SiC substrate 1 is flat beforethe removal of a work-affected layer due to the balance of stresses, thebalance of stresses is lost by the removal of the work-affected layerand, therefore, conversely a warp occurs. Because at this time, the SiCsubstrate 1 is not bonded to the substrate holder of the reactive ionetching device, the SiC substrate 1 can be held according to a change inthe warp.

[0053] That is, even if the SiC substrate is warped, by performing theremoval of a work-affected layer by the reactive ion etching process, itis possible to remove the work-affected layer from the surfacesubstantially uniformly and it is possible to hold the SiC substrate byallowing a change in the warp of the SiC substrate 1 which occurs inassociation with the removal of the work-affected layer. As a result ofthis, it is possible to eliminate the warp of the substrate and tosimultaneously achieve high parallelism and small thickness variations.Incidentally, during reactive ion etching, the chemical species of anetching gas in a plasma state collide with the SiC substrate 1 and thismay damage the surface of the SiC substrate 1. As described above, ithas been considered that the damage to the substrate surface by such aplasma is undesirable. In preferred embodiments of the presentinvention, however, this damage does not pose a problem. This isbecause, as will be described below, a principal surface on which awork-affected layer to be removed by the reactive ion etching process ispresent is not the surface on which an epitaxial layer is caused to growas the substrate and because the surface region of the SiC substrate inwhich damage occurs is to be removed later by the step of mirrorpolishing.

[0054] Thus, preferred embodiments of the present invention provide aunique advantage in that a work-affected layer is removed by reactiveion etching. And by combining the step of removing a work-affected layerby reactive ion etching with the step of polishing a SiC substrate, aSiC substrate having characteristics which previously have beenincapable of being obtained can be fabricated.

[0055] As steps capable of being combined with the step of removing awork-affected layer in preferred embodiments of the present invention,the above-described mechanical flattening and mirror polishing can beused. As mirror polishing, it is possible to use chemical mechanicalpolishing (CMP) which is accompanied by chemical etching. Chemicalmechanical polishing can remove a surface region of the substrate andreduce the surface roughness of the surface, with scarcely any new workstrains being generated. For this reason, unlike mechanical flattening,a new work-affected layer is scarcely formed during chemical mechanicalpolishing and the thickness of a work-affected layer is very small evenif it is formed. Therefore, the effect of compressive stresses by awork-affected layer are almost negligible. Furthermore, a surfacesubjected to chemical mechanical polishing becomes a mirror surface. Asurface finished to a mirror state has a surface roughness Ra of notmore than about 1 nm. Although generally colloidal silica is used inchemical mechanical polishing, other materials for chemical mechanicalpolishing may be used.

[0056] The method of manufacturing a SiC substrate according topreferred embodiments of the present invention will be described infurther detail below. Incidentally, in each of the drawings of FIGS. 4Ato 4D, FIGS. 7A to 7C and FIGS. 8A to 8C, finishing symbols are shownfor the principal surfaces of the substrates in order to indicate thesurface roughness.

[0057] First Preferred Embodiment

[0058] As shown in FIG. 4A, a SiC substrate 1 is prepared. As describedby referring to FIGS. 1 and 2, the SiC substrate 1 is cut from an ingot2 of SiC by cutting by use of a wire saw or other suitable cuttingapparatus. Work-affected layers 3 a and 3 b are formed on a firstprimary surface 1 a and a second primary surface 1 b of the SiCsubstrate 1, respectively, by cutting.

[0059] First, the first primary surface 1 a and the second primarysurface 1 b are polished by use of an appropriate abrasive or lappingdevice so that the first primary surface 1 a and the second primarysurface 1 b of the SiC substrate 1 obtain surface roughnesses that aresmaller than the surface roughness obtained by cutting. As a result ofthis, as shown in FIG. 4B, a portion of the work-affected layers 3 a, 3b of the first primary surface 1 a and the second primary surface 1 b isremoved.

[0060] Next, by subjecting the second principal surface 1 b in which thework-affected layers 3 b remains to chemical mechanical polishing, thework-affected layers 3 b are completely removed. The second principalsurface 1 b is a surface on which a semiconductor layer or other layersare to be formed later and a semiconductor element is to be formed. As aresult of this, as shown in FIG. 4C, a second principal surface 11 bfinished to a mirror-polished state is formed. Because on the side ofthe first principal surface 1 a the work-affected layer 3 a remains asit is, the SiC substrate 1 is warped as a whole so that the firstprincipal surface 1 a becomes concave.

[0061] Next, the work-affected layer 3 a remaining on a surface that isopposite to a surface on which a semiconductor element is to be formedis removed by reactive etching. Reactive etching is performed with theSiC substrate 1 held on a substrate holder within a reactive etchingdevice so that the second principal surface 11 b faces downward, wherebythe work-affected layer 3 a is completely removed. Because at this timethe second principal surface 11 b is in contact with the substrateholder, the second principal surface 11 b is not etched in the least.

[0062] The warp of the SiC substrate 1 comes to be eliminated as theabove-described work-affected layer 3 a is uniformly removed as a whole,and the work-affected layer 3 a is completely removed. Then, as shown inFIG. 4D, a substantially flat SiC substrate 11 with less warp isobtained. The surface roughness of the first principal surface ismaintained before and after etching. For this reason, a first principalsurface 11 a which is formed after the removal of the work-affectedlayer 3 a has a surface roughness of the same degree as the surfaceroughness by mechanical flattening. By lastly cleaning the SiC substrate11, the flat SiC substrate 11 in which only one side is finished to amirror state is obtained.

[0063] As described above, the first principal surface of the SiCsubstrate 11 has a surface roughness of a degree that can be obtained bymechanical flattening. More specifically, the surface roughness Ra ofthe first principal surface 11 a is about 10 nm to about 1 μm. On theother hand, the second principal surface 11 b is finished to amirror-polished state and has surface roughness Ra of not more thanabout 1 nm. Furthermore, the flatness of the whole SiC substrate iswithin about ±20 μm in the case of a substrate having a diameter ofabout 2 inches. Although in this preferred embodiment the firstprincipal surface is subjected to mechanical flattening, the firstprincipal surface may be kept in an as-cut state depending on theapplication of the substrate.

[0064] A substrate in which only one surface is mirror finished in thismanner according to this preferred embodiment has the advantage that,for example, in semiconductor manufacturing equipment, theidentification of the front surface and back surface of a substrate canbe easily performed and the advantage that because light scatters on asurface which is not mirror finished and hence light is not transmittedby this surface, exposure can be performed by use of an exposure deviceeven when the substrate material is transparent to a light source.

[0065] According to the conventional techniques, it is very difficult tomanufacture a SiC substrate in which only one surface is mirrorfinished. This is because it is necessary to perform chemical physicalpolishing in order to remove a work-affected layer and because surfaceroughness is necessarily reduced by chemical physical polishing. Forthis reason, a conventional SiC substrate in which only one surface ismirror finished inevitably has the work-affected layer on a surface thatis opposite to the mirror finished surface, and the warp of theconventional SiC substrate due to the work-affected layer is not lessthan about 60 μm.

[0066] Incidentally, in the step of reactive etching of this preferredembodiment, as shown in FIG. 5, the etching of a work-affected layer 3 ais performed by holding a SiC substrate 1 so that a second primarysurface 11 b, which is the surface on which a semiconductor element isto be formed, is opposed to a substrate holder 20 of the reactiveetching device. Because at this time the substrate holder 20 is alsoexposed to an etching gas, in some combinations of a gas which composesthe substrate holder 20 and the etching gas, the substrate holder 20 maybe etched and a contaminant 20′, such as substances composing the etchedsubstrate holder 20, may adhere to an area near an outer periphery 11 eof the second primary surface 11 b of the SiC substrate 1. Because thesecond primary surface 11 b is the surface on which a semiconductorelement is to be formed, it is undesirable that such a contaminant 20′should adhere to an area near the outer periphery 11 e of the secondprimary surface 11 b.

[0067] Therefore, when the contaminant 20′ has adhered, it is desirableto remove the contaminant 20′ after reactive etching. It is desirable toremove the contaminant 20′ by wet etching by using a solution which doesnot substantially dissolve the SiC substrate 1, but dissolves thecontaminant 20′ so as not to etch the SiC substrate 1 or cause damage tothe SiC substrate 1. That is, it is desirable to use an etching solutionwhich does not substantially dissolve the SiC substrate 1 and tofabricate the substrate holder 20 from a material which is readilydissolved by this etching solution.

[0068] In this preferred embodiment, although the first principalsurface 11 a has surface roughness of such an extent that can beobtained by mechanical flattening, the first principal surface 11 a mayalso be mirror finished by further performing chemical physicalpolishing. In this case, because there is no work-affected layer on thesurface of the first principal surface 11 a, the polishing time can beshortened compared to a case where polishing is performed by use ofconventional techniques. Because no warp occurs in the SiC substrate 11,there is no fear of worsening of the parallelism and a warp of the SiCsubstrate 11 by mirror finishing.

[0069] In this preferred embodiment, it is not always necessary that thestep of performing reactive etching be performed after the mirrorfinishing of the second principal surface 11 b. For example, after theSiC substrate 1 is cut by cutting, first the work-affected layer 3 a maybe removed by reactive etching.

[0070] Second Preferred Embodiment

[0071] In the same manner as with the first preferred embodiment, a SiCsubstrate 1 is prepared as shown in FIG. 6A. Work-affected layers 3 aand 3 b are formed on a first primary surface 1 a and a second primarysurface 1 b of the SiC substrate 1, respectively, by cutting ormechanical flattening.

[0072] First, the work-affected layers 3 a and 3 b present on the firstprimary surface 1 a and the second primary surface 1 b are completelyremoved by reactive etching. For example, with the SiC substrate 1 heldon a substrate holder within a reactive etching device so that thesecond principal surface 1 b is opposed to the substrate holder so as toallow a change in the warp of the substrate, reactive etching isperformed, whereby the work-affected layer 3 a is completely removed. Asdescribed in the first preferred embodiment, the work-affected layer 3 ais uniformly etched as a whole by reactive etching. Because a differenceis produced in the thickness of the work-affected layers 3 a and 3 b asthe thickness of the work-affected layer 3 a decreases, a difference instress is generated and a warp occurs in the SiC substrate 1 so that thesecond principal surface 1 b becomes convex. Next, the SiC substrate 1is reversed and the work-affected layer 3 b is removed. The differencein stress decreases with decreasing thickness of the work-affected layer3 b and the warp of the substrate is removed. As a result of this, asshown in FIG. 6B, a SiC substrate 1′ in which there is no work-affectedlayer in the first principal surface 1′a or the second principal surface1′b is obtained. Because in the SiC substrate 1′ there is nowork-affected layer in the two principal surfaces, scarcely any warpoccurs in the SiC substrate 1′.

[0073] Next, the second principal surface 1′b is subjected to chemicalmechanical polishing and finished to a mirror state. As a result ofthis, as shown in FIG. 6C, a SiC substrate 11 having a mirror-likesecond principal surface 11 b is obtained. Because there is no remainingwork-affected layer, no warp occurs in the SiC substrate 11 and in thecase of a substrate having a diameter of about 2 inches, the flatness iswithin about ±20 μm.

[0074] Incidentally, as required, the surface roughness of the firstprincipal surface 1 a may be reduced by performing the chemicalmechanical polishing of the first principal surface 1′a. According tothis preferred embodiment, although the first principal surface 1′a hasa surface roughness which is large enough to be obtained by cutting ormechanical flattening, there is no work-affected layer. For this reason,the surface roughness of the first principal surface 1 a can be adjustedby performing chemical mechanical polishing which does not form a newwork-affected layer for an arbitrary time.

[0075] Third Preferred Embodiment

[0076] A SiC substrate 1 is prepared (FIG. 7A) by following a proceduresimilar to that of the second preferred embodiment, and work-affectedlayers 3 a and 3 b are removed by reactive etching. As a result of this,as shown in FIG. 7B, a SiC substrate 1′ which is substantially flat andhas no work-affected layer is prepared. A first principal surface 1 aand a second principal surface 1′b of the SiC substrate 1′ have asurface roughness of such an extent that can be obtained by cutting.

[0077] Next, by use of a lapping device in which a bottom surface platehas a concave curved surface and a top surface plate has a convex curvedsurface, with the SiC substrate 1′ held so that the second principalsurface 1′b comes into contact with the bottom surface plate, the firstprincipal surface 1′a and the second principal surface 1′b aresimultaneously subjected to chemical mechanical polishing. As a resultof this, a SiC substrate 12 has a second principal surface 12 b that hasconvexity and a first principal surface 12 a that has concavity. Thatis, the obtained SiC substrate 12 is curved in such a manner that thesecond principal surface 12 b which is mirror finished is convex.

[0078] In this manner, usually a work-affected layer is not uniformlyformed for a surface which is formed by mechanical flattening orcutting. Therefore, if the next process is performed with awork-affected layer kept present, it is difficult to control the shapeof a substrate because of the presence of compressive stresses by thework-affected layer. According to the method of preferred embodiments ofthe present invention, however, because a work-affected layer is removedbeforehand, flatness, parallelism, shape, and other characteristics andparameters can be freely controlled by appropriately selecting the shapeof surface plates of a lapping device and the working method. Forexample, it is possible to fabricate a substrate which has a mirrorfinished convex surface and the surface that is opposite to this convexsurface is flat like a satin finished surface, a substrate in which thefront and back surfaces have a substantially parallel curved shape, asubstrate in which the two surfaces are concave surfaces, etc.

EXPERIMENTAL EXAMPLES

[0079] In the first preferred embodiment as shown in FIG. 4C, for theSiC single crystal substrate 1 having the second principal surface 11 bwhich is mirror finished, the work-affected layer 3 a was etched byreactive ion etching from the side of the first principal surface 1 aand the relationship between the etched amount and the parallelism ofthe SiC substrate 1 was investigated.

[0080] The second principal surface 11 b of the SiC single crystalsubstrate 1 having a diameter of about 2 inches is mirror finished andits surface roughness Ra is not more than about 0.3 nm. The firstprincipal surface 11 a is worked to provide a satin finished surface andits surface roughness Ra is not more than about 0.3 μm.

[0081] A parallel flat plate type reactive ion etching device is usedfor etching and the input power during etching is about 1.0 W/cm².Etching was performed by introducing CF₄ as a reactive gas into achamber at a flow rate of about 100 sccm and keeping the degree ofvacuum at about 2.0×10⁻³ torr. Parallelism was measured on the side ofthe second primary surface 11 b.

[0082]FIG. 8 is a graph showing the relationship between the etchedamount and the parallelism of the substrate. As shown in FIG. 8, theflatness of the SiC substrate is about −100 μm before etching (theetched amount: 0 μm). This shows that as shown in FIG. 4C, the SiCsubstrate 1 is warped so that the second principal surface 11 b becomesconcave.

[0083] As shown in FIG. 8, when the work-affected layer begins to beetched, flatness decreases abruptly. The flatness becomes not more thanabout ⅓ when etching is performed in an amount of about 1 μm. Theimprovement in flatness is not observed any more when etching isperformed in an amount of about 2.8 μm. In the case of this experimentalexample, it is apparent that the work-affected layer can be almostcompletely removed by etching the SiC substrate by not less than about2.5 μm.

[0084] Incidentally, although in the above-described preferredembodiments and experimental examples, the work-affected layer wascompletely removed by the reactive ion etching process, it is alsopossible to remove only a portion thereof by the reactive ion etchingprocess and to remove the remainder by chemical mechanical polishing.

[0085] The step of removing a work-affected layer by reactive ionetching, the step of mechanical flattening and the step of mirrorpolishing may be performed for one surface or both surfaces of the SiCsubstrate in orders other than those shown in the above-describedpreferred embodiments. By removing a work-affected layer withoutchanging the surface roughness of a worked surface, it is possible tocontrol various types of processing in the method of manufacturing a SiCsubstrate by polishing.

[0086] Thus, according to preferred embodiments of the presentinvention, a work-affected layer formed on a SiC substrate can be easilyremoved at a practical etching rate. Therefore, a flat SiC substrate canbe easily manufactured. Furthermore, because a work-affected layer canbe removed with scarcely any change in the surface roughness of a workedsurface, it is also possible to manufacture a substrate in which onlyone surface is mirror finished. It is possible to use an obtained SiCsubstrate in a preferable manner as a substrate for formingsemiconductor layers, such as high-quality GaN-base semiconductorlayers, SiC semiconductor layers, and as a dummy wafer used in thesemiconductor manufacturing process.

[0087] The present invention is not limited to each of theabove-described preferred embodiments, and various modifications arepossible within the range described in the claims. An embodimentobtained by appropriately combining technical features disclosed in eachof the different preferred embodiments is included in the technicalscope of the present invention.

What is claimed is:
 1. A method of manufacturing a SiC substrate whichhas a first principal surface and a second principal surface, comprisingthe step of removing, by a vapor phase etching process, at least aportion of a work-affected layer which is formed by mechanicalflattening or cutting on the first principal surface of the SiCsubstrate.
 2. The method of manufacturing a SiC substrate according toclaim 1, wherein the vapor phase etching process is a reactive ionetching process.
 3. The method of manufacturing a SiC substrateaccording to claim 1, wherein the second principal surface is a surfacewhere an element is to be formed.
 4. The method of manufacturing a SiCsubstrate according to claim 1, further comprising the step of mirrorpolishing the second principal surface.
 5. The method of manufacturing aSiC substrate according to claim 1, wherein the SiC substrate has awork-affected layer which is formed by mechanical flattening or cutting,on the second principal surface, and the method further comprises thesteps of: removing at least a portion of the work-affected layer of thesecond principal surface by a vapor phase etching process; and mirrorpolishing at least the second principal surface after the steps ofremoving are performed.
 6. The method of manufacturing a SiC substrateaccording to claim 1, wherein the SiC substrate has a work-affectedlayer which is formed by mechanical flattening or cutting, on the secondprincipal surface, and the method further comprises the step of removingthe work-affected layer of the second principal surface by mechanicalpolishing and chemical mechanical polishing and mirror finishing thesecond principal surface.
 7. The method of manufacturing a SiC substrateaccording to claim 1, wherein the first principal surface obtained bythe step of removing has a surface roughness of about 10 nm to about 1μm.
 8. The method of manufacturing a SiC substrate according to claim 1,wherein the method further includes a step of cutting the SiC substratefrom an ingot of SiC and the first principal surface and secondprincipal surface are formed by the step of cutting.
 9. The method ofmanufacturing a SiC substrate according to claim 1, wherein in the stepremoving, the SiC substrate is held so as to allow a change in the warpof the SiC substrate.
 10. The method of manufacturing a SiC substrateaccording to claim 1, wherein a gas containing fluorine is used in thevapor phase etching process.
 11. The method of manufacturing a SiCsubstrate according to claim 10, wherein the gas containing fluorine isCF₄ or SF₆.
 12. The method of manufacturing a SiC substrate according toclaim 10, wherein in the vapor phase etching process, the work-affectedlayer is removed at an etching rate in a range of about 0.5 μm/hr toabout 20 μm/hr.
 13. The method of manufacturing a SiC substrateaccording to claim 10, wherein the SiC substrate is one of amorphous, apoly crystal and a single crystal.
 14. A SiC substrate manufactured bythe manufacturing method specified in claim
 1. 15. A SiC substratecomprising two substantially parallel principal surfaces, wherein onlyone of the two principal surfaces is mirror finished and the warp is notmore than about ±50 μm.